The escalating requirements for high densification and performance associated with ultra large scale integration semiconductor devices requires design features of 0.25 microns and under, such as 0.18 microns and under, increased transistor and circuit speeds, high reliability and increased manufacturing throughput for competitiveness. The reduction of design features to 0.25 microns and under challenges the limitations of conventional semiconductor manufacturing technology.
The aggressive scaling of gate electrode dimensions into the deep submicron regime, such as less than about 0.25 microns, demands extremely shallow junctions to maintain good short channel characteristics and current drive. For example, semiconductor devices having design features of about 0.25 microns require a significantly reduced lightly doped drain (LDD) junction depth (X.sub.J) of less than about 800.ANG.. Conventional methodology comprises ion implanting an N-type impurity (N) having a low diffusion coefficient, typically arsenic (As). The formation of a sharp N-LDD junction requires the power supply voltage (V.sub.dd) to be reduced in order to maintain sufficient hot carrier reliability. While attractive from a power dissipation standpoint, a lower V.sub.dd compromises speed and current drive required for microprocessors particularly for desktop applications. Thus, hot carrier injection (HCI) reliability has become the limiting factor for performance of N-channel MOSFETs, particularly as the design rules shrink. A reduction in the HCI lifetime is attributed to the sharp N-LDD junction which causes a high peak electric field in the channel region.
D. Nayak et al., in "A Comprehensive Study of Performance and Reliability of P, As, and Hybrid As/P N-LDD Junctions for Deep-Submicron CMOS Logic Technology," IEEE Electron Device Letters, Vol. 18, No. 6, 1997, pp. 281-283, disclose a method of N-LDD junction grading to decrease the peak electric field in the channel, thereby improving the HCI lifetime. The disclosed technique comprises ion implanting As and P to form the N-LDD implant. While this technique was reported to improve the HCI lifetime in semiconductor devices having 0.35 micron technology, such a hybrid As/P-LDD technique cannot be directly applied to semiconductor devices having design features of about 0.25 microns and below, because the Off-current is increased to an unacceptably high level.
In copending application Ser. No. 08/924,644, a method is disclosed comprising ion implanting a rapidly diffusing N-type impurity, such as P, into a doped (As) source/drain implant after sidewall spacer formation. Upon activation annealing, a graded N-LDD junction is formed with an attendant significant increase the HCI lifetime without an increase in the Off-current. In copending application Ser. No. 08/979,364, a method is disclosed comprising plural ion-implantations (As) at different dosages and different angles to the semiconductor substrate. Upon activation annealing, a graded N-LDD junction is formed with an attendant increase in the HCI lifetime.
In copending application Ser. No. 08/924,640, filed on Sep. 5, 1997, a method is disclosed for independently forming source/drain regions of a P-channel and N-channel transistor of a CMOS semiconductor device employing plural sidewall spacers for independent control of the respective lengths of the channel regions.
In high performance oriented integrated circuits, such as those employed in microprocessors, capacitance loading must be reduced to as great an extent as possible to avoid reductions in circuit speed. Accordingly, there exists a need for semiconductor methodology and devices having a reduced junction capacitance. There exists an even greater need for semiconductor methodology and devices having a design rule of about 0.25 microns and under having reduced junction capacitance.